Only probe for DNS64 presence when we know that we can talk to the slaacd

Only probe for DNS64 presence when we know that we can talk to the slaacd
provided nameservers, i.e. the stub resolver check succeeded.
Previously we would only probe DNS64 on network change but would not
reschedule when it failed. Sometimes (most of the time?) this failes
because our address is still tentative or a default route has
not yet been installed.
OK phessler